4. We need more tests. Many more tests

While the Vikings and later rovers have been a success, we've only sampled the smallest part of the planet.
While I crave the vicarious glory of a human mission to mars, a robotic blitz needs to come first.
We can get a half dozen rovers with as many separate experiments onto the Martian ground for the weight of a single EVA suited astronaut.
And we can get them there with existing rockets. Round trips to Mars with human crew
requires rather more and more capable heavy lift rockets than we currently posses.
We'll get the most information for our money from one way instrument packages.
A sample return would be a cool achievement.
But I submit that experiments across the Martian Surface would yield more information than kilo or so of material we might get back with a return.
I'm sure trying to reproduce the first Viking results with similar instruments scattered across Mars would produce useful results.
We've all heard of sensor chips. Now our manufacturers can strut their nanobot stuff to produce micro experiment packages.
A few Mars Orbiters could give yield a mountain of data by remote sensing.